Method and device for controlling firing circuits for restraining devices

ABSTRACT

A device and a method are proposed for controlling firing circuits for an element of restraint, individual transistors of the output stages being controlled by bit combinations, and impermissible bit combinations lead to an error message to a processor. In addition, a firing current is measured, in order to estimate a firing energy from it, so that efficient energy management can be carried out. Furthermore, determination of the firing current makes it possible to update a crash protocol as to whether the firing circuit was activated or not. When a voltage of the energy reserve is exceeded, it is further proposed to switch over to pulse operation for operating the output stages. Thereby a greater resistance to short-circuits and a higher efficiency are achieved.

FIELD OF THE INVENTION

[0001] The present invention relates to a device and a method forcontrolling firing circuits for restraining devices.

BACKGROUND INFORMATION

[0002] In air bags, integrated switching circuits for controlling firingcircuits are already being used, wherein single bits in a firing commandcontrol single transistors in output stages. Firing commands that arenot recognized are ignored, and there is no reaction.

SUMMARY OF THE INVENTION

[0003] The device according to the present invention and the methodaccording to the present invention for controlling firing circuits forrestraining devices have the advantage, compared to that, that by usinga bit combination, each controllable transistor is clearly identified,as a result of which a bit error does not create false signals bymistake. When a bit error occurs, the error recognition of the firingcontrol recognizes it and notifies the processor. It is also ofadvantage that a faulty processor cannot release all output stagesbecause of the inhibiting inputs present for the plus and minus outputstages and the corresponding enabling commands. Beyond that, it is ofadvantage that the firing current is now clearly measurable, so that, onaccount of that, a notification on the functionability of therestraining devices is possible. The switchover between a pulseoperation and a continuous operation of the output stages also leads toa higher firing speed or rather, higher firing efficiency.

[0004] It is of particular advantage that the element for measuring thefiring current registers a current of a reference current source inconnection with the adjustment information to a predefined desiredcurrent in a plus output stage, in each case, and stores the respectivemeasuring result in a firing current register, so that this measuringresult can then be transmitted to a crash recorder. The current sourceis, for instance, a current mirror in the plus output stage. Inlawsuits, the record of the firing circuit's activity is of advantage asproof. Furthermore, measuring the firing current to determine the firingenergy is of advantage since, when the minimum firing current is known,the minimum firing energy can be calculated from this firing current,the resistance of the firing circuit and the firing time. This, then,makes possible intelligent energy management for the purpose ofswitching off the output stage transistors for firing of the firingelement as soon as these are no longer needed, that is, when the minimumfiring energy has already been reached. That also means that a smallercapacitor can be used as energy store for the energy reserve.

[0005] In addition, it is of advantage that the firing circuit controlsends an error message to the processor, if a faulty bit combination wasreceived, which activates no transistor, so that the processor can reactaccordingly, and can check its own functionability. This increases thesafety of the whole system.

[0006] Again, it is of advantage that a switchover between pulsedoperation and continuous operation of the output stages is carried outas a function of the energy reserve voltage, at high voltages highcurrent firing being carried out, namely using pulsed operation, andthis increasing firing efficiency and firing speed. The firingefficiency increases based on the higher voltage at the firing element,brought on by the high current firing—at simultaneously lower voltagedrop at the output stage. In the case of currently used pyrotechnicalfiring elements, the triggering time decreases rapidly with increasingfiring current, whereby firing speed is raised. Using pulsed operation,the plus output stage becomes short-circuit-resistant to short-circuitto ground, since from the pulsing at the output stage transistor only alimited power is created, which is smaller than that which is maximallypermissible. Beyond that, in pulsed operation it is possible to havehigher firing currents without thermal overload of the plus outputstage. In addition, the area of the plus output stage on an integratedcircuit can be made smaller.

BRIEF DESCRIPTION OF THE DRAWING

[0007]FIG. 1 shows a block diagram of the device according to thepresent invention.

[0008]FIG. 2 shows a block diagram of the firing control circuit.

[0009]FIG. 3 shows a flow diagram of the method according to the presentinvention.

DETAILED DESCRIPTION

[0010] Air bags are increasingly being applied in motor vehicles inlarger numbers and having greater intelligence. The firing of air bagsis effected by a firing circuit control which is usually present on anintegrated circuit. It is the task of the firing circuit control toactivate the transistors of the output stages correctly, namely as afunction of firing instructions which come from a processor of the airbag control unit, to monitor the firing current, and optimally to managethe energy available for firing.

[0011] Therefore, according to the present invention, a device and amethod for controlling firing circuits for restraint devices isproposed, the device having the appropriate arrangement for performingthese tasks, and in the method, in particular a bit combination beingused for controlling the individual transistors; in case of a bit error,the bit combination not being about to activate any other transistor, sothat no erroneous activation of an output stage will occur, because abit error of a permissible bit combination automatically leads to animpermissible bit combination.

[0012]FIG. 1 shows the block diagram of the device according to thepresent invention. A processor 1 of an air bag control device isconnected to a firing circuit control 2 via a first data input/output, asafety IC (integrated circuit) 40 as well as a firing current register11. Firing circuit control 2 itself represents an integrated circuit,that is, an IC. Processor 1 is connected to a firing current register 11via a data input/output. Safety IC 40 is connected to a first inhibitinginput of firing current control 2 via a first data output, this firstinhibiting input being used to suspend minus output stages of firingcircuits.

[0013] Safety IC 40 is connected to a second inhibiting input of firingcurrent control 2 via a second data output, this second inhibiting inputbeing used to disable plus output stages of the firing circuits. A plusoutput stage 5 is connected to an energy reserve 8 for voltage supply,which has essentially at least one capacitor as an energy store. Thisenergy reserve 8 is used for igniting a firing element connected tooutput stages 5 and 7. Energy reserve 8 is additionally connected tofiring circuit control 2 for registering the energy reserve voltage.Firing circuit control 2 is connected to a minus output stage 7 via afirst output. Firing circuit control 2 is connected to a plus outputstage 5 via a second output. Firing circuit control 2 is connected toplus output stage 5 via an input. On the other side of plus output stage5 a firing element 6 is connected, which is also connected on the otherside to minus output stage 7. Plus output stage 5 has a currentlimitation, while minus output stage 7 does not have this. Firingcircuit control 2 is connected to a second data input/output of firingcurrent register 11, via a second data input/output. Processor 1 writeson crash recorder 12. Here there is only one plus output stage or minusoutput stage, as the case may be, but it is possible to provide severalpairs of plus and minus output stages, to the extent that there arecorrespondingly more firing circuits present.

[0014] Firing element here especially means a priming cap.

[0015] Safety IC 40 and processor 1, which are located in a controldevice of the air bag, are connected to sensors via data connections notshown here, in order to be able to recognize a triggering case for theway of restraint independently of one another. Such sensors may be, forexample, acceleration sensors or pressure sensors for sensing sideimpact. Safety IC 40 fulfills the function of plausibility checking ofthe sensor values, and recognizes from data in its own hardware whethera triggering case exists or not. In a triggering case, safety IC 40releases the output stages. Safety IC is thus like a safety switch,which, however, reacts to sensor values from all directions.

[0016] In the triggering case, then, the air bags are fired as elementsof restraint. Then processor 1 transmits a firing command to firingcircuit control 2 via data connection 9 which activates varioustransistors of output stages 5 and 7. However, safety IC 40 first setsthe disable inputs 3 and 4 with appropriate enable commands, so thatoutput stages 5 and 7 are each enabled. Then processor 1 transmits thefiring command by activating the transistors. The following bitcombinations are used, for example. Bit combination 01 means that anoutput stage transistor is connected, and bit combination 10 means thatthe respective output stage transistor is disconnected. Bit combinations00 and 11 are not permissible. The command is then refused accordingly.Firing circuit control 2 will subsequently set an error bit, in order tosignal this error to processor 1, which is a microcontroller here. Byuse of these bit combinations, it is made clear that when a bit erroroccurs, the bit combinations 01 and 10 become impermissible commands.That is, the error is recognized. Both bits have to be in error for bitcombination 01 and 10 so as to lead to a faulty reaction. In this case,if a data capacity of 8 bits is used, four transistors of the outputstages can be controlled by the four bit pairs. The bit pairs are thenpermanently assigned to the transistors. For that, data line 9 isappropriately designed.

[0017] Firing circuit control 2 then correspondingly controls thetransistors of plus output stage 5 and minus output stage 7. As anexample, only one plus output stage and one minus output stage each areconnected here. However, substantially more plus output stages and minusoutput stages, and thus also firing circuits, can be connected. Firingcircuit control 2 then registers, via data input 10, a reference currentof a current mirror in plus output stage 5 and the adjustmentinformation of plus output stage 5 to the desired current. Theadjustment information is a digital signal, such as a bit or a bitcombination which indicates that a predefined threshold current wasreached. Both quantities, the reference current and the adjustmentinformation are linked logically in order to estimate the firingcurrent. Here, an AND link is selected for the logical linking. Thecurrent mirror is used here, for instance, as a regulated currentsource. If this firing current reaches a minimum predefined thresholdvalue, firing circuit control 2 sets firing control register 11. Thisfiring current register 11 is read out cyclically by processor 1, andthe content is permanently stored in crash recorder 12. This is ofinterest for subsequent lawsuits, so as to be able to prove thefunctionality of the restraint system. It is further possible toestimate the firing energy with the minimum firing current. The firingenergy is calculated from the following formula:

Energy=I_(min)*I_(min)*firing circuit resistance*firing time

[0018] With that, after the firing energy sufficient for firing has beenreached, the output stage of the software can be switched off, so as notto discharge the energy reserve capacitor in energy reserve 8unnecessarily.

[0019] The available plus output stages are either pulsed or activatedin continuous operation as a function of the energy reserve voltage inenergy reserve 8. If a predefined voltage threshold for the energyreserve voltage is exceeded, efficiency-optimized pulse operation takesplace, i.e. pulse output stage 5 is automatically switched on and off bya predefined, fixed pulse/pause ratio. If the voltage lies below it,continuous operation is applied. The switching change is doneautomatically. A firing mode once selected, that is, either pulseoperation or continuous operation, remains so for the duration of thefiring, independently of the further course of the energy reservevoltage. Using pulse operation, plus output stage 5 becomesshort-circuit-resistant to short-circuits to ground. In addition, inpulse operation greater firing currents are possible without thermaloverload of plus output stage 5.

[0020] In FIG. 2 firing circuit control 2 is shown in detail in a blockdiagram. At inhibiting inputs 3 and 4, in each case logic circuits arepresent on the IC of firing circuit control 2, which cause the releaseor inhibiting of plus output stage 5 or minus output stage 7,respectively. For this purpose, function block 15 is connected to afunction block 16 which carries out the control of the plus outputstage. Function block 14, however, is connected to function block 17,which controls the minus output stage. Function blocks 16 and 17 thenprocess the pertinent enabling commands.

[0021] Function block 13 is connected to data connection 9, over whichthe control commands are transmitted. The control commands are heretransmitted via the so-called serial peripheral interface (SPI), thetransmission being sequenced in so-called SPI data frames. Functionblock 13 evaluates whether the received control bit combinations arepermissible or not. If an impermissible value appears, processor 1 isnotified in the next SPI data frame by setting an error bit. Thus,function block 13 is connected to function block 16 via a first dataoutput, and to function block 17 via a second data output in order tocause the appropriate control of the transistors of plus and minusoutput stages 5 and 7. Function block 18, to which data are transmittedvia input 10, carries out the above-described firing currentregistration by measuring a reference current in a regulated currentsource, here a current mirror, of the plus output stage. In addition,the adjustment information of the plus output stage to desired currentis logically linked to the presence of the reference current. If thisfiring current exceeds a minimum desired current, function block 18transmits this result to firing current register 11, so as to set therea corresponding bit when the minimum firing current is exceeded. Thefiring energy can be calculated from the minimum firing current, inorder then to switch off the output stages so as to save energy, usingprocessor 1.

[0022] A function block 20 is connected to energy reserve 8 formeasuring the energy reserve voltage of energy reserve 8. If this energyreserve voltage is less than a predefined value, then function block 20,which is connected to function block 16 via a data connection, controlsfunction block 16 in such a way that pulse operation of the plus outputstages is present. This pulse operation is efficiency-optimized andpermits higher firing currents, without giving rise to thermal overloadof the plus output stage.

[0023] A function block 20 is connected to energy reserve 8 for thepurpose of measuring the energy reserve voltage of energy reserve 8. Ifthis energy reserve voltage is greater than a predefined value, functionblock 20, which is connected to function block 16 via a data connection,switches function block 16 so that pulse operation of the plus outputstages is present. This pulse operation is efficiency-optimized andpermits larger firing currents, without giving rise to thermal overloadof the plus output stages.

[0024] In FIG. 3, the method according to the present invention isrepresented as a flow chart. In method step 21, safety IC 40 andprocessor 1 have recognized from sensor values that the restraintsystems should be fired, and that they should send a firing command tofiring circuit control 2. In method step 22 it is determined whichoutput stages are to be activated.

[0025] Activation of each output stage then begins in method step 23.For this purpose, inhibiting inputs 3 and 4 are first set to a lowpotential by safety IC 40. In method step 24, processor 1 releases plusand minus output stages 5 and 7 by giving two enabling commands. Inmethod step 25, the activating commands, which are put into effect bythe above-described bit combinations, are transmitted to firing control2, via data connection 9. The activation of output stages 5 and 7 isonly possible by maintaining this sequence, otherwise output stages 5and 7 would automatically be disabled again.

[0026] In method step 26, function block 13 checks whether the bitcombinations are permissible or not. If the bit combinations arepermissible, then in method step 27 activating the output stages isperformed by function blocks 16 and 17. If the bit combinations were notpermissible, then in method step 28, processor 1 is notified via dataconnection 9 that the activating commands are faulty. In method step 29,which follows method step 32 or 33, flow of the minimum firing currentin plus output stage 5 is then determined by function block 18. Theresult is then transmitted by function block 18 to firing currentregister 11. The firing energy can be ascertained from the firingcurrent. As soon as the firing energy has reached a minimum value, plusoutput stage 5 and minus output stage 7 are switched off by processor 1,in order to save energy. In addition, the voltage of energy reserve 8 ischecked. This is done in method step 30. If the voltage of the energyreserve is greater than a predefined value, function block 20 controlsfunction block 16 in such a way that plus output stage 5 is operated inpulse operation. This is checked in method step 31, in order to applypulse operation, if necessary, in method step 32, and, in case thevoltage is less than the predefined threshold, to use continuousoperation in method step 23.

What is claimed is:
 1. A device for controlling firing circuits forelements of restraint in a motor vehicle, comprising: a processor; afiring circuit control; a plurality of firing elements; a plurality ofoutput stages for each of the plurality of firing elements, each outputstage being in communication with a respective inhibiting input forreleasing an associated one of the plurality of output stages; an energyreserve for an operation of the firing circuits, the firing circuitsincluding the plurality of output stages and the plurality of firingelements, wherein: the processor releases the plurality of output stagesfor triggering the elements of restraint when a crash of the motorvehicle occurs; and a safety IC for releasing the plurality of outputstages during the crash and including an arrangement for performing acrash recognition, wherein the firing circuit control includes: anarrangement for evaluating an activating command, an arrangement foroperating the plurality of output stages, an arrangement for measuring afiring current, an arrangement for handling an error in the activatingcommand, and an arrangement for measuring an energy reserve voltage andfor switching between a pulse operation and a continuous operation forthe plurality of output stages, the safety IC setting each inhibitinginput after a start-up of the device.
 2. The device according to claim1, further comprising: a crash recorder; and a firing current register,wherein: the arrangement for measuring the firing current registers acurrent of a current source, in each case, in one of the plurality ofoutput stages and stores a measuring result in the firing currentregister, the measuring result then being transmittable to the crashrecorder.
 3. The device according to claim 2, wherein: the arrangementfor measuring the firing current logically links a reference current ofa regulated current source of the one of the plurality of output stagesand an adjustment information, so as to determine whether the firingcurrent is present.
 4. The device according to claim 1, wherein: thearrangement for handling the error in the activating command transmitsan error message to the processor when the arrangement for handling theerror in the activating command recognizes the activating command asbeing faulty.
 5. The device according to claim 3, wherein: for aregistration of a firing energy, the processor uses the measuring resultfrom the firing current register for ascertaining the firing energy, andthe processor switches off the plurality of output stages by using thefiring circuit control when the firing energy reaches a predefinedvalue.
 6. The device according to claim 1, wherein: the arrangement formeasuring the energy reserve voltage and for switching between the pulseoperation and the continuous operation registers the energy reservevoltage and compares the energy reserve voltage to a threshold value,and the arrangement for measuring the energy reserve voltage and forswitching between the pulse operation and the continuous operationswitches to the pulse operation when the threshold value is exceeded,and otherwise to the continuous operation.
 7. The device according toclaim 1, wherein: the safety IC disables the plurality of output stagesby setting each respective inhibiting input.
 8. A method for controllingfiring circuits for elements of restraint, in which output stagesinclude transistors, the method comprising the steps of: causing aprocessor to activate the firing circuits via a firing circuit control;in order to achieve a control of the processor, providing at least twobits as a bit combination for controlling one of the transistors; andsignaling an error message to the processor by the firing circuitcontrol when an impermissible bit combination occurs.
 9. The methodaccording to claim 8, further comprising the step of: activating one ofthe output stages by performing a sequence that includes: placinginhibiting inputs of the firing circuit control for the output stages ata predefined voltage level by the safety IC, causing the processor tosend a release command for each one of a plurality of function blocks,and causing the firing circuit control to receive a bit combination foractivating the one of the output stages from the processor.
 10. Themethod according to claim 9, wherein: when the sequence is notmaintained, an automatic breaking off of the activating of the one ofthe output stages occurs.